New Grain Refiners and Their Refining Behaviors of AZ31 Magnesium Alloy
|Keywords||AZ31 magnesium alloy ε-AlMn phase ZnO nano-rods AlCo phase grain size grain refinement|
Magnesium alloys have been considered as green engineering materials in 21st century, and thus extensively developped in the the industrial fields of automobiles,3C products, bicycles, and so on. As compared to the other metal materials such as Al or Fe alloys, however, the application of magnesium alloys is relatively limited because of its inherent properties, such as low strength, poor corrosion resistance and insufficient high-temperature performance. Therefore, it is practically important to refine grain sizes of the as-cast magnesium alloys to improve the comprehensive properties and to decrease the manufacture costs. The addition of grain refiners to melt is the simplest and cheapest one among all the grain refining methods of Mg alloys. Unfortunately, there have not been any effective refiners which can be adopted to the most popular Mg-Al based alloys. In this work, therefore, the effects of the Al-Mn, Al-Co master alloys and ZnO nanorods on the grain size of AZ31 Mg alloy have been studied by means of X-Ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Optical Microscopy (OM), and it is expected to provide the valuable information for further exploiting the refining mechanism of the the new grain refiners for the Mg-Al based alloys and thus improving their comprehensive mechanical properties with low cost.So far, there has not been one international or national standard for characterizing the refining effect of Mg-based alloys. It is widely accepted by researchers that Mg alloys are generally solution treated at 400℃for 12-24 hours and then to characterize their grain sizes. In this work, the grain boundaries ofα-Mg matrix can be clearly identified when the as-cast AZ31 Mg alloy was held at 400℃for 4h. which is adopted in this work and significantly improves the experimental efficiency. The effect of the holding time in liquid state of AZ31 Mg alloy on the grain size is not obvious at the moulding temperature of 750℃. It suggests that the grain size of AZ31 Mg alloy at one holding time can be used to compare with that after the addition of the various refiners at different holding times.The Al-63wt.% Mn、Al-66wt.% Mn and Al-74wt.% Mn master alloys are composed of single Al8Mn5 phase, Al5Mn5 andε-AlMn phases, and singleε-AlMn phase, respectively. The experiments show that the grain sizes of AZ31 Mg alloy can be refined only for the master alloy containing theε-AlMn phase, i.e., the Al-66wt.% Mn and Al-74wt.% Mn alloys. Moreover, the effect of the content of s-AIMn phase on the grain size is not obvious. It implies that the refining mechanism of the Al-Mn master alloys is due to the heterogeneous nucleation of theε-AlMn phase, whose structure is same as that ofα-Mg phase with a small lattice misfit. When the Al-74 wt.% Mn alloy prepared by arc melting or single Cu roller melt spinning, is added to the AZ31 Mg alloy, respectively, the refining efficiency has little difference. It implies that the two preparation methods above of the Al-Mn alloy containing theε-AlMn phase do not affect of the refining efficiency of AZ31 Mg alloy. The grain size of AZ31 cast Mg alloy is remarkably refined with the addition of Al-74wt.% Mn alloy (0.5 wt.%) when held at the moulding temperature of 750℃for 40 minutes, i.e., decreased from 90μm to 53μm.When the Al-70wt.% Co master alloy, which is composed of single AlCo phase, is added to the AZ31 magnesium alloy, the grain refinement is also remarkable. Moreover, the refining efficiency is hardly affected by the adding amount of Al-70 wt.% Co alloy after its weight content is larger than 1 wt.%. The most refining efficency is obtained with the addition of Al-70wt.% Co alloy(1 wt.%) and the holding time of 60 minutes at the moulding temperature of 750℃, i.e., decreased from 90μm to 45μm. When the Al-70 wt.% Co alloys prepared by arc melting or single.Cu roller melt spinning, is added to the AZ31 Mg alloy, respectively, the refining efficiency has little difference, which implies that the two preparation method above of the Al-Co alloy containing the AICo phase does not affect the refining efficiency of AZ31 Mg alloy. Because the crystal structure of AlCo phase is BCC, different from theα-Mg phase, and the smallest misfit value is about 12% which is between <11 20>Mg and<100> AlCo, the refining mechanism of the AlCo phase on AZ31 magnesium alloy needs to be further studied in the future.The ZnO nano-rods have been prepared by the homogeneous precipitation method, whose diameter is～100 nm with an aspect ratio of～10. The grain refinement is also considerably obvious after the ZnO nano-rods is added to the AZ31 Mg alloy, especially with the addition from 0.3 wt.% to 1 wt.%. The optimal refining efficency is obtained with the addition of the ZnO nano-rods (1 wt.%) and the holding time of 3 min at the moulding temperature of 720℃, i.e., decreased from 90μm to 49μm. The grain refinement is hardly affected by the holding time. The refining mechanism of ZnO nanorods is ascribed to the heterogeneous nucleation by the ZnO nano-rods, whose structure is same as that ofα-Mg phase, with a low lattice misfit of about 0.59%.In summary, the grain size of the AZ31 Mg alloy can be effectively refined by the addition of either the Al-Mn master alloy containing theε-AlMn phase, Al-Co master alloy containing the AlCo phase or ZnO nanorods, with grain sizes of AZ31 alloy decreased by 41%,50% and 45%, respectively. The hardness of the AZ31 magnesium alloy is also improved by the either addition of the three kinds of the refiners above for both the as-cast and homogenized states, respectively.